To find the perimeter of the triangle we find the lengths of each of the sides. To find the length, use the distance formula: between points `(x_1,y_1),(x_2,y_2) ` the distance `d ` is given by `d=sqrt((x_2-x_1)^2+(y_2-y_1)^2) ` . Note that the distance formula is just the Pythagorean theorem.